My son's interview for visa has scheduled on next week. He reached age of 21 in 2 weeks after form I-130 was accepted.

communicate with the consulate that this is an age out case.

Missing the immigration inspection officers. What is their schedule like?

If they want to meet with you they will let you know. Most likely they will talk to your neighbors and confirm you both live there.

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I am an unmarried 34 years old foreign child born from a foreign born US mother, am i a US citizen ?

If your parents became U.S. citizens before your 18th birthday then you can look at the requirements to see if you derived citizenship.

Can you apply for OPT Stem extension on expired F1 visa?

yes you can. The visa is only applicable if you leave the U.S.

it may take months until you will know. Did you request special consideration for late filing and give a good reason? If your are going to worry...

Before you go back to China be sure your travel permit is approved otherwise you will be abandoning your I-485.

It applies to I-130's and fiancee petitions. In or out of the U.S.

Is any immigration attorney able to take my case to an immigration court and help me renew my EAD there upon its expiration?

You should retain a qualified immigration attorney in the San Francisco area.

Is it going to cause some problems during green card process if I go work remotely from home (within US)?

You will need to file a new LCA and amended H-1B to work in a location not listed on the original lCA.

N400 Interview cancelled

Nothing to worry about -- it is not uncommon. They will send you an official notice cancelling and then another notice when it is rescheduled.